The present invention relates to a method of manufacturing a collector for a microwave tube such as a traveling-wave tube or a klystron.
A microwave tube such as a traveling-wave tube or a klystron is used in various fields of communication, television broadcasting, radar, industrial heating, nuclear fusion, and the like. In recent years, the microwave tube has been important. Especially, in the field of satellite communication, a microwave tube having high-efficiency, high-frequency, and high-output characteristics is demanded. As an amplifier in this field, a microwave tube such as a traveling-wave tube or a klystron is demanded. In a recently desired microwave tube to be mounted in a satellite, an increase in efficiency of the microwave tube is the most important problem because the number of microwave tubes mounted in a satellite is limited.
A beam rectilinear type microwave tube generally amplifies and oscillates a microwave using an electron beam. FIGS. 3A and 3B show a traveling-wave tube which is a typical beam rectilinear type microwave. This traveling-wave tube is mainly constituted by an electron gun unit 4, a high-frequency circuit unit 5, and a collector unit 6. Hot electrons are emitted from a cathode 7 of the electron gun unit 4, accelerated by a grid and an anode, and incident on the high-frequency circuit unit 5. In the high-frequency circuit unit 5, an incident electron beam 8 interacts with a high-frequency signal input from an input portion 9, and this high-frequency signal is amplified and extracted from an output portion 10. In the collector unit 6, the electron beam 8 which interacts with the high-frequency signal is captured, and the kinetic energy of the electron beam 8 is converted into heat energy.
In order to increase the efficiency of such a traveling-wave tube, the recovery efficiency of the electron beam 8 must be increased in, especially, the collector unit 6. Various conventional methods of increasing the recovery efficiency are provided. In a conventional collector, as shown in FIGS. 4A and 4B, a collector electrode has a multi-stage collector structure constituted by a first collector 62, a second collector 63, and a third collector 64, and a secondary electron preventing film 12 such as a graphite film, a titanium nitride film, or a titanium carbide film having a low secondary electron emissivity is formed on the surfaces of each collector. In the multi-stage collector structure, assuming that the potentials of the electrodes of the first collector 62, the second collector 63, and the third collector 64 are represented by Vc1, Vc2, and Vc3, respectively, and that the circuit voltage of the traveling-wave tube is represented by Vs, voltages are applied to the collectors to satisfy Vs&gt;Vc1&gt;Vc2&gt;Vc3. The electron beam 8 which interacts with a high-frequency signal in the high-frequency circuit unit 5 is classified into electrons having different speeds by decelerating electric fields generated by the first collector 62, the second collector 63, and the third collector 64 and a diverging force generated by space charges. More specifically, the slowest electrons, the second slowest electrons, and the fastest electrons are incident on and captured by the first collector 62, the second collector 63, and the third collector 64, respectively.
The recovery rate of an electron beam by such a multi-stage potential gradient collector can be increased in proportion to the number of stages of the collector electrode. In practice, a collector having 2 to 4 stages is popularly used. In addition, each collector electrode captures the electrons of an electron beam which is incident on the collector electrode, and, at the same time, the collector electrode generates secondary electrons on its surface. These secondary electrons are accelerated toward a high-potential portion. As indicated by broken arrows in FIGS. 3B and 4A, when a larger number of reversely traveling electrons 11 are generated, the reversely traveling electrons 11 adversely affect the distortion characteristics of the traveling-wave tube to increase a helix current inside the high-frequency circuit. This causes the traveling-wave tube to vary in output so as to considerably impair the function of the traveling-wave tube.
For this reason, in a conventional technique, a graphite powder having a low secondary electron emissivity is coated on the surface of the collector electrode described above, and the resultant structure is used in practice. However, according to this method, a carbon powder is generated by vibration of a traveling-wave tube, an ion impact, or the like to decrease the operation efficiency of the traveling-wave tube. Therefore, the method cannot be properly applied to a high-output traveling-wave tube.
Note that a technique for coating a graphite film, a titanium nitride film, or a titanium carbide film on the surface of a collector electrode by a CVD (Chemical Vapor Deposition) method or a PVD (Physical Vapor Deposition) method to improve the adhesion properties of the secondary electron preventing film 12 is disclosed in Japanese Patent Laid-Open No. 63-939. Although these films are excellent in adhesion properties, each film has a relatively flat surface state, and the secondary electron preventing effect of each film is limited. Therefore, even when the above methods are used, an increase in efficiency of a traveling-wave tube is limited.
As a recent technique for improving a surface quality, a composite plating technique as shown in FIG. 5 is disclosed in Japanese Patent Laid-Open No. 2-213498 or 2-118080. According to this technique, hard powder particles 13 consisting of boron nitride, graphite, and the like are dispersed in a metal plating solution, and a composite plating layer 14 containing these hard particles 13 dispersed in a metal plating layer is precipitated on the surface of a metal material, thereby improving the surface characteristics (mainly, friction characteristics and lubrication characteristics) of a matrix. However, even when the dispersed particles are precipitated by the conventional composite plating technique, the degree of exposure and the surface area of the dispersed particles are not sufficient with respect to the secondary electron preventing effect of a metal part.